Conventionally, various thermal-sensitive color developing materials that develop a color upon heat application, and various thermal-sensitive elements using the same are known. For example, Japanese Laid-Open Publication No. 7-149057 discloses a thermal-sensitive element 150 shown in FIG. 15 in which an air layer 154 and an organic layer 153 are sequentially provided on a colored layer 155. The organic layer 153 contains a material which reversibly changes the scattering of light depending on the temperature. Specifically, such a material is made of a resin (e.g., a vinyl chloride resin) and a low molecular weight organic substance dispersed therein. The low molecular weight organic substance may be a material which reversely changes between a transparent state and an opaque state depending on the temperature. Such a low molecular weight organic substance is usually transparent at room temperature and becomes opaque at a temperature of about 70-80.degree. C. or higher. Accordingly, in such a thermal-sensitive element 150, when the organic layer 153 is in the transparent state at a predetermined temperature, the color of the colored layer 155 is identifiable through the organic layer 153. Furthermore, when the organic layer 153 is in the opaque state at a temperature different from the above-mentioned predetermined temperature, the color of the colored layer 155 is screened by the organic layer 153, thereby the color being unidentifiable through the organic layer 153. The thermal-sensitive element having such a structure can be utilized as a display element which changes its color tones depending on the temperature by using a mechanism of transmitting and screening the color of the colored layer 155 by the organic layer 153.
However, since the above-described thermal-sensitive element 150 utilizes transition between the transparent state and the opaque state of the organic layer 153 depending on the temperature, it is extremely difficult to color the whole thermal-sensitive element in various colors and moreover, a colored layer must always be provided. In addition, since the transparent/opaque states of the low molecular weight organic substance used in the thermal-sensitive element 150 reversibly transit depending on the temperature, the thermal-sensitive element 150 may be preferably used, for example, as a display element which changes its color tones depending on the temperature. However, it cannot be used as a element for displaying temperature history indicating that the temperature has once risen to induce the opaque state, and thereafter fallen to recover the transparent state. Furthermore, in such a thermal-sensitive element 150, since the transition to the opaque state initiates at a relatively high temperature, a change in the temperature cannot be checked in the case where a product, such as a frozen food or a drug, that usually needs to be stored at a relatively low temperature (for example, about -20.degree. C. to about 4.degree. C.) has been exposed, e.g., to about room temperature.
Japanese Laid-Open Publication No. 61-110585 discloses a thermal-sensitive element having a thermal-sensitive color developing layer and a protective layer sequentially provided on a support. The thermal-sensitive color developing layer which contains a leuco pigment, developer and a zirconium compound, reversibly develops color at a temperature of about 50.degree. C. or higher. As is the case of the above-described thermal-sensitive element, such a thermal-sensitive element, too, cannot be used as an element for displaying temperature history indicating that the temperature has once risen to induce a colorless state, and thereafter fallen to recover the color developed state.
Japanese Laid-Open Publication No. 60-171191 discloses a packaging material provided with a thermal-sensitive portion which can be subjected to thermal printing at the time of shipping or distribution of a packaged item. Such a packaging material is fabricated by applying, printing or adhering a thermal-sensitive color changing material as a thermal-sensitive portion on a surface of a packaging film (for example, polypropylene).
However, since the color of the above-described thermal-sensitive color changing material changes only when exposed at a high temperature (for example, about 200.degree. C.), a change in the temperature cannot be checked in the case where a product, such as a frozen food or a drug, that needs to be stored at a relatively low temperature has been exposed, e.g., to about room temperature.